cause they ended with a ceasefire treaty. not peace, but ceasefire. meaning they arent supposed to shoot at each other for no good reason.
and regarding the torpedo, i believe it was mentioned in ST that it was a human torpedo, or something along that line. where it is actually quite small, pretty much undetectable to begin with
http://en.wikipedia.org/wiki/Human_torpedo
http://en.wikipedia.org/wiki/Midget_submarine
when u blow a torpedo, i believe its only the tip end that has explosives. the propellor shld be relatively undamaged on the other end.
either way, today ST seems to imply sg stance that Nkorea did it. i'm waiting to see china's development, for if they do find the findings conclusive, which i hope they do, it would certainly carry more weight, seeing they are allies of Nkorea.